Planned astronomical observatories of the 2020s will be capable of obtaining
reflected light photometry and spectroscopy of cool extrasolar giant planets.
Here we explain that such data are valuable both for understanding the origin
and evolution of giant planets as a whole and for preparing for the
interpretation of similar datasets from potentially habitable extrasolar
terrestrial planets in the decades to follow.Comment: Science white paper submitted to the Astro 2020 Decadal Survey on
  Astronomy and Astrophysics. Replace version to fix typo in co-signer name and
  add figure credit

Arney, Giada

Bailey, Vanessa

Batalha, Natasha

Beichman, Charles

Benneke, Björn

Blecic, Jasmina

Cahoy, Kerri

Chilcote, Jeffrey

Domagal-Goldman, Shawn

Dressing, Courtney

Fitzgerald, Michael

Fortney, Jonathan

Freedman, Richard

Gelino, Dawn

Gizis, John

Greene, Thomas

Guyon, Olivier

Hammel, Heidi

Hasegawa, Yasuhiro

Jovanovic, Nemanja

Konopacky, Quinn

Kopparapu, Ravi

Lewis, Nikole

Liu, Michael

Lopez, Eric

Lunine, Jonathan

Lupu, Roxana

Macintosh, Bruce

Mandt, Kathleen

Marley, Mark

Marois, Christian

Mawet, Dimitri

Mayorga, Laura

Morley, Caroline

Nielsen, Eric

Roberge, Aki

Serabyn, Eugene

Skemer, Andrew

Stapelfeldt, Karl

Visscher, Channon

Wang, Jason

English

arXiv

Planned astronomical observatories of the 2020s will be capable of obtaining reflected light photometry and spectroscopy of cool extrasolar giant planets. Here we explain that such data are valuable both for understanding the origin and evolution of giant planets as a whole and for preparing for the interpretation of similar datasets from potentially habitable extrasolar terrestrial planets in the decades to follow

Caltech Authors - Main

Astro2020 Science White Paper 
Imaging Cool Giant Planets in Reflected 
Light: Science Investigations and Synergy 
with Habitable Planets 
Thematic Area:                  ☒ Planetary Systems     
Principal Author:  
Name: Mark Marley       
Institution: NASA Ames Research Center 
Email: Mark.S.Marley@NASA.gov  
Phone: (650) 604-0805 
Co-authors: 
Nikole Lewis (Cornell Univ.) and Giada Arney (GSFC), Vanessa Bailey (JPL), Natasha 
Batalha (UCSC), Charles Beichman (NESI), Björn Benneke (U. Montreal), Jasmina 
Blecic (NYUAD), Kerri Cahoy (MIT), Jeffrey Chilcote (U. Notre Dame), Shawn Domagal-
Goldman (GSFC), Courtney Dressing (UC Berkeley), Michael Fitzgerald (UCLA), 
Jonathan Fortney (UCSC),  Richard Freedman (ARC), Dawn Gelino (NESI), John Gizis 
(U. Deleware), Olivier Guyon (U. Arizona), Thomas Greene (ARC), Heidi Hammel 
(AURA), Yasuhiro Hasegawa (JPL), Nemanja Jovanovic (Caltech), Quinn Konopacky 
(UCSD), Ravi Kopparapu (GSFC), Michael Liu (U. Hawaii), Eric Lopez (GSFC), Jonathan 
Lunine (Cornell), Roxana Lupu (BAER), Bruce Macintosh (Stanford), Kathleen Mandt 
(JHU), Christian Marois (U. Victoria), Dimitri Mawet (Caltech), Laura Mayorga (CfA), 
Caroline Morley (U.T. Austin), Eric Nielsen (Stanford), Aki Roberge (GSFC), Eugene 
Serabyn (JPL), Andrew Skemer (UCSC), Karl Stapelfeldt (JPL), Channon Visscher 
(Doredt Univ.), Jason Wang (Caltech) 
Abstract: 
Planned astronomical observatories of the 2020s will be capable of obtaining 
reflected light photometry and spectroscopy of cool extrasolar giant planets. Here we 
explain that such data are valuable both for understanding the origin and evolution of 
giant planets as a whole and for preparing for the interpretation of similar datasets 
from potentially habitable extrasolar terrestrial planets in the decades to follow. 
 1
Introduction  
WFIRST and the extremely large telescopes with specialized instruments, such as 
TMT/PSI or GMT/GMagAO-X, planned for the next decade will have the capability to 
detect and characterize giant planets in reflected light. Here we argue that scattered 
light spectroscopy of giant planets offers outstanding science opportunities and should 
be considered an important component of the exoplanet science portfolio in the next 
decade for two reasons: (1) such data will provide scientifically valuable new insights 
into extrasolar planets; and (2) these planets provide a critical training ground for 
developing and testing methods for interpreting scattered light spectra of extrasolar 
planets in preparation for LUVOIR, HabEx, or the WFIRST + Starshade mission. 
Exoplanet Scattered Light Spectroscopy  
Modeling and interpretation of directly-imaged planets in scattered light is 
intrinsically different from the analysis of transiting planets that has comprised much 
of exoplanet science to this point. Incident rays detected in transit spectra pass 
through an atmosphere at fairly low pressures where, once scattered, there is 
generally a very low chance of photons being scattered back into the beam which 
reaches the observer (see Robinson 2017). This somewhat forgiving set of 
circumstances has helped fuel the boom in exoplanet transit spectroscopy.  
The interpretation of photons received from the partially illuminated disk of a planet 
as observed by direct imaging is more complex. Reflected light observations are 
sensitive to a multitude of scattering pathways through the atmosphere. Photons 
arriving at the limb are often primarily singly scattered from high-altitude hazes, 
usually produced by photochemistry, while photons returning from the center of the 
disk have probed much deeper, by factors of 100 or more in pressure, into the 
atmosphere, and encountered multiple cloud layers (e.g., Batalha et al. 2019). 
Necessarily, the abundance of atmospheric gases of interest (e.g., CH4 or H2O) must 
be derived along with the entire atmospheric haze, cloud, and thermal structure in 
order to interpret the reflected light spectrum of a planet. This is because the 
measured depths of gaseous absorption bands depend upon both the continuum flux 
level, set by cloud or gas scattering, and the column of absorbing gas above the cloud 
layer, which in turn depends on the thermal profile and gravity of the planet. Unlike 
for the case of transiting planets, which usually have measured mass and radius – and 
thus gravity and scale height – as prior constraints, there will be few to no constraints 
on radius for directly-imaged planets, compounding the challenge (GAIA should 
provide masses for many known RV planets). Given such challenges there is a need for 
proving grounds where observing strategies and analysis algorithms can be tested on 
easier targets than potentially habitable planets.  
Cool Giant Planet Science  
The first extrasolar planet to be detected around a main sequence star was a hot 
Jupiter, 51 Peg b, with an equilibrium temperature, Teq, of over 1200 K. However, 
despite the subsequent stunning successes of this field, essentially all transiting 
planets since characterized have Teq > 500 K. The observed atmosphere in these cases 
 2
is strongly impacted by incident flux and there is a deep radiative zone which acts to 
decouple the dynamics and chemistry of the observed atmosphere from the deeper 
interior. Only for cooler, more distant giant planets—which have never been probed by 
transit methods—do we expect atmospheric thermal profiles to be more like our own 
Solar System giant planets, with an observable atmosphere directly coupled to the 
deep interior.  
Because of this distinction between the hot Jupiters and cooler giants, we do not 
know if various trends identified among the transiting planets hold for all giants. For 
example much effort has focused on determining if there is a mass-metallicity trend 
in hot Jupiter atmospheres that is comparable to that seen in solar system giants 
(where metallicity falls with increasing mass, e.g., Wakeford et al. 2018). For giant 
planets somewhat warmer than our own Jupiter, H2O, CH4, and NH3 will all be present 
in gaseous form, allowing measurement of C, N, and O abundances as a function of 
planetary mass and orbit, stellar metallicity, and other parameters. Only by directly 
observing cool, distant extrasolar giants will a truly direct comparison be possible.  
Furthermore, because of the role cool giants planets may play in delivering volatiles 
to potentially habitable planets (e.g., Marov & Ipatov 2018), the record of this process 
preserved in their own atmospheric abundances, and the great diversity of 
atmospheric chemistry, cloud processes, photochemistry, and dynamics likely present 
in their atmospheres, the cool giants (with masses ranging from ‘sub-Neptune’ to 
‘super-Jupiter’) are worthy of detailed characterization.  
There are many long orbital period giants from radial velocity searches which are 
excellent targets for space-based coronagraphs (Figure 1) and more will be found by 
future direct imaging surveys. Some characterization of these planets may occur 
serendipitously by missions focusing on habitable planet discovery and 
characterization. However only purposeful surveys will collect sufficient data to 
permit an understanding of the comparative planetology of cool giant planets. We 
Figure 1: Collection of known RV planets 
detectable by direct imaging in reflected 
light from space (blue) and the ground (red, 
dots scaled by planet mass) in the next 
decade. Open circles show the population of 
planets to be characterized by JWST in 
transmission (including expected TESS 
discoveries). This transit population is 
distinct from the direct imaging targets, 
favoring hotter planets. Cloud icons denote 
the planet Teﬀ ranges over which various 
major species condense, sculpting the 
reflected light spectra (Figure 2). Eﬀective 
temperature estimates account for both 
absorption of flux from the star and the 
planets’ own internal energy. Figure courtesy 
N. Lewis.
 
 3
urge that any future direct imaging missions recognize the importance of cool giant 
planet science and plan accordingly for such science to be accommodated, e.g., 
through Design Reference Missions, instrument design, selection of participating 
scientists, operational development, and so on. Without mindful instrument design 
and operation choices, giant exoplanet science could be precluded.  
Giants as a Training Ground  
In addition to their intrinsic science value, giants are independently useful as 
calibration and training targets for the effort to characterize potentially habitable 
planets. Specifically, the complex multiple-scattering atmosphere codes required to 
properly interpret the reflected-light datasets that will be obtained by future direct 
imaging missions generally do not exist within the exoplanet community today. These 
must be constructed and tested without the common simplifications that are 
appropriate for the analysis of exoplanet transit spectra. Reflected-light spectra of 
potentially habitable terrestrial planets will almost certainly have low signal-to-noise 
ratios (SNR), contain elements of cloud and photochemical haze scattering as well as 
gaseous absorption, and consequently be challenging to interpret. The community 
needs assurance that the interpretation methods and models used to study the data 
are robust. Likewise the expertise among astronomers necessary to plan and interpret 
reflected-light observations must be nurtured. Most exoplanet astronomers today 
have little to no experience with such datasets. 
Fortunately, giant-planet reflected-light spectra will offer an excellent, high SNR 
training ground to develop observational and theoretical methods to interpret such 
reflected-light data for planets (Marley et al. 1999; Sudarsky et al. 2003, 2005; Cahoy 
et al. 2010). Bulk composition, equilibrium chemistry, cloud composition, and 
photochemical pathways are relatively well understood for giant planets. Thus, 
forward models and retrieval methods face fewer unknowns than they do for 
terrestrial planets (Venus, Earth, and Mars exhibit far greater atmospheric diversity 
than Jupiter, Saturn, and Neptune, for example) and are natural testing grounds for 
observational interpretation methods such as spectroscopic retrievals. They are also a 
control sample to confirm that habitability false positives do not unexpectedly 
appear, perhaps from unanticipated photochemical pathways for example. Giant 
planets can therefore serve as high quality targets for the interpretation of reflected 
light data before these same techniques are applied to terrestrial planets.  
Giant Planet Science Investigations  
Below we briefly highlight three areas where giants provide interesting intrinsic 
science and can enhance direct imaging habitable planet science.  
Composition  
The atmospheres of cool gas giant planets are predominantly H2-He with a few 
percent sprinkling of other notable gasses, predominantly CH4, NH3, H2O, and – for 
warm planets – detectable Na and K. These gases and their condensates sculpt the 
optical reflected-light spectra. What is of interest to observers and theorists alike is 
 4
the abundance of these gases relative to that of their primary stars. In the solar 
system, Jupiter is enhanced by a factor of about 4 and Saturn a factor of 10 in C for 
example. The specific fingerprint of gas abundances is understood to provide insight 
into the reservoir of material that impacted the growing planets during their 
formation (Fortney et al. 2013; Lunine et al. 2019). Early trends in transiting planets 
show some evidence that the metallicity enhancement may be inversely proportional 
to planet mass. Whether such trends continue for colder giants that potentially 
formed farther from their stars and did not migrate is of particular interest.  
The major species detected in reflected light spectra of the gas giant planets will 
almost certainly be those predicted by chemical equilibrium, as their atmospheres are 
dense and well mixed (trace amounts of disequilibrium species, such as CO or PH3, 
are unlikely to be detectable for the cool giants that will be characterized by direct 
imaging). Thus optical to near-IR spectroscopy has the potential to measure true C, N, 
and O abundances. Figure 2 shows some of the diversity of reflected light spectra 
expected from various cool giant planets. Methane abundance and cloud top pressure 
can be derived with appropriate measurements of such spectra (R > 50, SNR > 15, 
e.g., Lupu et al. 2016, Batalha et al. 2019). Tests of abundance retrieval techniques 
on the relatively well understood giant planets will also give confidence that such 
methods are valid when applied to potentially habitable terrestrial exoplanet 
atmospheres characterized by the same instruments (e.g., Feng et al. 2018). 
Clouds  
Clouds and hazes in planetary atmospheres both inside and outside of the solar system 
critically shape reflected-light spectra. The influence of clouds on planetary spectra is 
notoriously difficult to predict, and clouds are a leading source of uncertainty in 
terrestrial climate models. With their onion-skin-like atmospheric structure, giant 
Figure 2: Collection of model reflected light spectra of cool giant planets. Top three spectra show 
sensitivity to metallicity while bottom model shows the remarkable sensitivity to clouds. Figure 
courtesy Natasha Batalha.
 5
exoplanets offer a continuous array of cloud types to scatter incident flux. Figure 2 
highlights the impact of H2O clouds on the reflected-light spectrum of a giant planet 
somewhat warmer than Jupiter. In fact, giant exoplanets in the habitable zones well 
outside the tidal locking distance of their host stars will provide some of our first 
opportunities to understand the formation of water clouds and their effect on global 
energy budgets in exoplanet atmospheres. The robustness with which we are able to 
constrain the properties of clouds (e.g., cloud top pressures, degree of partial 
cloudiness, single scattering albedos, and scattering asymmetry factors) will critically 
impact our ability understand clouds and extract the planetary thermal and chemical 
structure of both terrestrial and giant planets.  
Photochemistry  
Photochemical processes also play key roles in shaping the atmospheres and observed 
spectra of all solar system planets. In the stratospheres of solar system giant planets, 
CH4 photochemistry generates hydrocarbons such as C2H6 and C2H4, and these can 
polymerize into more complex hydrocarbon species, some of which form aerosols. 
These hydrocarbons strongly absorb UV and blue light on Jupiter and Saturn. 
Hydrocarbon hazes are also crucial components of the atmosphere of Titan and likely 
were important at some periods in the atmosphere of the Archean Earth. In addition, 
giant exoplanets somewhat warmer than Jupiter will likely host an array of 
photochemical S, N, and O species. Lab results point to the likely ubiquity of 
photochemical hazes under diverse conditions (Hörst et al. 2018). Any complete 
characterization of an exoplanet atmosphere should account for the presence of 
hazes. Studies of hazy giant planets in reflected light would provide a valuable 
proving ground for understanding photochemical processes in atmospheres that differ 
from those found in the solar system and would give insight into habitable exoplanet 
haze processes.  
Understanding the diversity of photochemical outcomes for all types of planets is a 
challenge. Because they form at low stratospheric pressures and are small, haze 
particles are most apparent from their interaction with blue and UV light. 
Investigating how hazes relate to atmospheric thermochemical structure, and surface 
composition/processes for terrestrial planets, will be a complex task, but one that 
will greatly inform our understanding of such processes as a whole. 
Conclusions  
The path forward for characterization of potentially habitable worlds through direct 
imaging proposed here mimics in many ways the path that has developed in the study 
of transiting exoplanets. Observational strategies, data reduction and interpretation 
techniques, and atmospheric modeling efforts were greatly refined on a sizeable 
(>100) population of giant exoplanets before inroads were made for transiting 
terrestrial exoplanets. It is our hope that any future facilities that aim to study 
exoplanets in detail acknowledge the criticality of exploring and learning from giant 
planets along the path towards characterizing habitable terrestrial planets.  
 6
References  
Batalha, N., Marley, M., et al. (2019) Ap. J., submitted. 
Cahoy, K. et al. (2010) Ap. J. 724, 189.  
Feng, Y. et al. (2018) Astron. J. 155, 200.  
Fortney, J. et al. (2013) Ap. J. 775, 80.  
Hörst, S. et al. (2018) Nature Astron. 2, 343. 
Marley, M. et al. (1999) Ap. J. 513, 879.  
Marov, M. & Ipatov, S. (2018) Solar System Res. 52, 392. 
Lunine, J. et al. (2019) white paper to the 2020 Astronomy Decadal Survey. 
Lupu, R. et al. (2016) Ap. J. 152, 217.  
Robinson, T. (2017) Ap. J. 836, 236.  
Sudarsky, D. et al. (2003) Ap. J. 588, 1121.  
Sudarsky, D. et al. (2005) Ap. J. 627, 520.  
Wakeford, H. et al. (2018) Astron. J. 155, 29. 
 7


Imaging Cool Giant Planets in Reflected Light: Science Investigations and Synergy with Habitable Planets

Astro2020 Science White Paper 
Imaging Cool Giant Planets in Reflected 
Light: Science Investigations and Synergy 
with Habitable Planets 
Thematic Area:                  ☒ Planetary Systems     
Principal Author:  
Name: Mark Marley       
Institution: NASA Ames Research Center 
Email: Mark.S.Marley@NASA.gov  
Phone: (650) 604-0805 
Co-authors: 
Nikole Lewis (Cornell Univ.) and Giada Arney (GSFC), Vanessa Bailey (JPL), Natasha 
Batalha (UCSC), Charles Beichman (NESI), Björn Benneke (U. Montreal), Jasmina 
Blecic (NYUAD), Kerri Cahoy (MIT), Jeffrey Chilcote (U. Notre Dame), Shawn Domagal-
Goldman (GSFC), Courtney Dressing (UC Berkeley), Michael Fitzgerald (UCLA), 
Jonathan Fortney (UCSC),  Richard Freedman (ARC), Dawn Gelino (NESI), John Gizis 
(U. Deleware), Olivier Guyon (U. Arizona), Thomas Greene (ARC), Heidi Hammel 
(AURA), Yasuhiro Hasegawa (JPL), Nemanja Jovanovic (Caltech), Quinn Konopacky 
(UCSD), Ravi Kopparapu (GSFC), Michael Liu (U. Hawaii), Eric Lopez (GSFC), Jonathan 
Lunine (Cornell), Roxana Lupu (BAER), Bruce Macintosh (Stanford), Kathleen Mandt 
(JHU), Christian Marois (U. Victoria), Dimitri Mawet (Caltech), Laura Mayorga (CfA), 
Caroline Morley (U.T. Austin), Eric Nielsen (Stanford), Aki Roberge (GSFC), Eugene 
Serabyn (JPL), Andrew Skemer (UCSC), Karl Stapelfeldt (JPL), Channon Visscher 
(Doredt Univ.), Jason Wang (Caltech) 
Abstract: 
Planned astronomical observatories of the 2020s will be capable of obtaining 
reflected light photometry and spectroscopy of cool extrasolar giant planets. Here we 
explain that such data are valuable both for understanding the origin and evolution of 
giant planets as a whole and for preparing for the interpretation of similar datasets 
from potentially habitable extrasolar terrestrial planets in the decades to follow. 
 1
Introduction  
WFIRST and the extremely large telescopes with specialized instruments, such as 
TMT/PSI or GMT/GMagAO-X, planned for the next decade will have the capability to 
detect and characterize giant planets in reflected light. Here we argue that scattered 
light spectroscopy of giant planets offers outstanding science opportunities and should 
be considered an important component of the exoplanet science portfolio in the next 
decade for two reasons: (1) such data will provide scientifically valuable new insights 
into extrasolar planets; and (2) these planets provide a critical training ground for 
developing and testing methods for interpreting scattered light spectra of extrasolar 
planets in preparation for LUVOIR, HabEx, or the WFIRST + Starshade mission. 
Exoplanet Scattered Light Spectroscopy  
Modeling and interpretation of directly-imaged planets in scattered light is 
intrinsically different from the analysis of transiting planets that has comprised much 
of exoplanet science to this point. Incident rays detected in transit spectra pass 
through an atmosphere at fairly low pressures where, once scattered, there is 
generally a very low chance of photons being scattered back into the beam which 
reaches the observer (see Robinson 2017). This somewhat forgiving set of 
circumstances has helped fuel the boom in exoplanet transit spectroscopy.  
The interpretation of photons received from the partially illuminated disk of a planet 
as observed by direct imaging is more complex. Reflected light observations are 
sensitive to a multitude of scattering pathways through the atmosphere. Photons 
arriving at the limb are often primarily singly scattered from high-altitude hazes, 
usually produced by photochemistry, while photons returning from the center of the 
disk have probed much deeper, by factors of 100 or more in pressure, into the 
atmosphere, and encountered multiple cloud layers (e.g., Batalha et al. 2019). 
Necessarily, the abundance of atmospheric gases of interest (e.g., CH4 or H2O) must 
be derived along with the entire atmospheric haze, cloud, and thermal structure in 
order to interpret the reflected light spectrum of a planet. This is because the 
measured depths of gaseous absorption bands depend upon both the continuum flux 
level, set by cloud or gas scattering, and the column of absorbing gas above the cloud 
layer, which in turn depends on the thermal profile and gravity of the planet. Unlike 
for the case of transiting planets, which usually have measured mass and radius – and 
thus gravity and scale height – as prior constraints, there will be few to no constraints 
on radius for directly-imaged planets, compounding the challenge (GAIA should 
provide masses for many known RV planets). Given such challenges there is a need for 
proving grounds where observing strategies and analysis algorithms can be tested on 
easier targets than potentially habitable planets.  
Cool Giant Planet Science  
The first extrasolar planet to be detected around a main sequence star was a hot 
Jupiter, 51 Peg b, with an equilibrium temperature, Teq, of over 1200 K. However, 
despite the subsequent stunning successes of this field, essentially all transiting 
planets since characterized have Teq > 500 K. The observed atmosphere in these cases 
 2
is strongly impacted by incident flux and there is a deep radiative zone which acts to 
decouple the dynamics and chemistry of the observed atmosphere from the deeper 
interior. Only for cooler, more distant giant planets—which have never been probed by 
transit methods—do we expect atmospheric thermal profiles to be more like our own 
Solar System giant planets, with an observable atmosphere directly coupled to the 
deep interior.  
Because of this distinction between the hot Jupiters and cooler giants, we do not 
know if various trends identified among the transiting planets hold for all giants. For 
example much effort has focused on determining if there is a mass-metallicity trend 
in hot Jupiter atmospheres that is comparable to that seen in solar system giants 
(where metallicity falls with increasing mass, e.g., Wakeford et al. 2018). For giant 
planets somewhat warmer than our own Jupiter, H2O, CH4, and NH3 will all be present 
in gaseous form, allowing measurement of C, N, and O abundances as a function of 
planetary mass and orbit, stellar metallicity, and other parameters. Only by directly 
observing cool, distant extrasolar giants will a truly direct comparison be possible.  
Furthermore, because of the role cool giants planets may play in delivering volatiles 
to potentially habitable planets (e.g., Marov & Ipatov 2018), the record of this process 
preserved in their own atmospheric abundances, and the great diversity of 
atmospheric chemistry, cloud processes, photochemistry, and dynamics likely present 
in their atmospheres, the cool giants (with masses ranging from ‘sub-Neptune’ to 
‘super-Jupiter’) are worthy of detailed characterization.  
There are many long orbital period giants from radial velocity searches which are 
excellent targets for space-based coronagraphs (Figure 1) and more will be found by 
future direct imaging surveys. Some characterization of these planets may occur 
serendipitously by missions focusing on habitable planet discovery and 
characterization. However only purposeful surveys will collect sufficient data to 
permit an understanding of the comparative planetology of cool giant planets. We 
Figure 1: Collection of known RV planets 
detectable by direct imaging in reflected 
light from space (blue) and the ground (red, 
dots scaled by planet mass) in the next 
decade. Open circles show the population of 
planets to be characterized by JWST in 
transmission (including expected TESS 
discoveries). This transit population is 
distinct from the direct imaging targets, 
favoring hotter planets. Cloud icons denote 
the planet Teff ranges over which various 
major species condense, sculpting the 
reflected light spectra (Figure 2). Effective 
temperature estimates account for both 
absorption of flux from the star and the 
planets’ own internal energy. Figure courtesy 
N. Lewis.
 
 3
urge that any future direct imaging missions recognize the importance of cool giant 
planet science and plan accordingly for such science to be accommodated, e.g., 
through Design Reference Missions, instrument design, selection of participating 
scientists, operational development, and so on. Without mindful instrument design 
and operation choices, giant exoplanet science could be precluded.  
Giants as a Training Ground  
In addition to their intrinsic science value, giants are independently useful as 
calibration and training targets for the effort to characterize potentially habitable 
planets. Specifically, the complex multiple-scattering atmosphere codes required to 
properly interpret the reflected-light datasets that will be obtained by future direct 
imaging missions generally do not exist within the exoplanet community today. These 
must be constructed and tested without the common simplifications that are 
appropriate for the analysis of exoplanet transit spectra. Reflected-light spectra of 
potentially habitable terrestrial planets will almost certainly have low signal-to-noise 
ratios (SNR), contain elements of cloud and photochemical haze scattering as well as 
gaseous absorption, and consequently be challenging to interpret. The community 
needs assurance that the interpretation methods and models used to study the data 
are robust. Likewise the expertise among astronomers necessary to plan and interpret 
reflected-light observations must be nurtured. Most exoplanet astronomers today 
have little to no experience with such datasets. 
Fortunately, giant-planet reflected-light spectra will offer an excellent, high SNR 
training ground to develop observational and theoretical methods to interpret such 
reflected-light data for planets (Marley et al. 1999; Sudarsky et al. 2003, 2005; Cahoy 
et al. 2010). Bulk composition, equilibrium chemistry, cloud composition, and 
photochemical pathways are relatively well understood for giant planets. Thus, 
forward models and retrieval methods face fewer unknowns than they do for 
terrestrial planets (Venus, Earth, and Mars exhibit far greater atmospheric diversity 
than Jupiter, Saturn, and Neptune, for example) and are natural testing grounds for 
observational interpretation methods such as spectroscopic retrievals. They are also a 
control sample to confirm that habitability false positives do not unexpectedly 
appear, perhaps from unanticipated photochemical pathways for example. Giant 
planets can therefore serve as high quality targets for the interpretation of reflected 
light data before these same techniques are applied to terrestrial planets.  
Giant Planet Science Investigations  
Below we briefly highlight three areas where giants provide interesting intrinsic 
science and can enhance direct imaging habitable planet science.  
Composition  
The atmospheres of cool gas giant planets are predominantly H2-He with a few 
percent sprinkling of other notable gasses, predominantly CH4, NH3, H2O, and – for 
warm planets – detectable Na and K. These gases and their condensates sculpt the 
optical reflected-light spectra. What is of interest to observers and theorists alike is 
 4
the abundance of these gases relative to that of their primary stars. In the solar 
system, Jupiter is enhanced by a factor of about 4 and Saturn a factor of 10 in C for 
example. The specific fingerprint of gas abundances is understood to provide insight 
into the reservoir of material that impacted the growing planets during their 
formation (Fortney et al. 2013; Lunine et al. 2019). Early trends in transiting planets 
show some evidence that the metallicity enhancement may be inversely proportional 
to planet mass. Whether such trends continue for colder giants that potentially 
formed farther from their stars and did not migrate is of particular interest.  
The major species detected in reflected light spectra of the gas giant planets will 
almost certainly be those predicted by chemical equilibrium, as their atmospheres are 
dense and well mixed (trace amounts of disequilibrium species, such as CO or PH3, 
are unlikely to be detectable for the cool giants that will be characterized by direct 
imaging). Thus optical to near-IR spectroscopy has the potential to measure true C, N, 
and O abundances. Figure 2 shows some of the diversity of reflected light spectra 
expected from various cool giant planets. Methane abundance and cloud top pressure 
can be derived with appropriate measurements of such spectra (R > 50, SNR > 15, 
e.g., Lupu et al. 2016, Batalha et al. 2019). Tests of abundance retrieval techniques 
on the relatively well understood giant planets will also give confidence that such 
methods are valid when applied to potentially habitable terrestrial exoplanet 
atmospheres characterized by the same instruments (e.g., Feng et al. 2018). 
Clouds  
Clouds and hazes in planetary atmospheres both inside and outside of the solar system 
critically shape reflected-light spectra. The influence of clouds on planetary spectra is 
notoriously difficult to predict, and clouds are a leading source of uncertainty in 
terrestrial climate models. With their onion-skin-like atmospheric structure, giant 
Figure 2: Collection of model reflected light spectra of cool giant planets. Top three spectra show 
sensitivity to metallicity while bottom model shows the remarkable sensitivity to clouds. Figure 
courtesy Natasha Batalha.
 5
exoplanets offer a continuous array of cloud types to scatter incident flux. Figure 2 
highlights the impact of H2O clouds on the reflected-light spectrum of a giant planet 
somewhat warmer than Jupiter. In fact, giant exoplanets in the habitable zones well 
outside the tidal locking distance of their host stars will provide some of our first 
opportunities to understand the formation of water clouds and their effect on global 
energy budgets in exoplanet atmospheres. The robustness with which we are able to 
constrain the properties of clouds (e.g., cloud top pressures, degree of partial 
cloudiness, single scattering albedos, and scattering asymmetry factors) will critically 
impact our ability understand clouds and extract the planetary thermal and chemical 
structure of both terrestrial and giant planets.  
Photochemistry  
Photochemical processes also play key roles in shaping the atmospheres and observed 
spectra of all solar system planets. In the stratospheres of solar system giant planets, 
CH4 photochemistry generates hydrocarbons such as C2H6 and C2H4, and these can 
polymerize into more complex hydrocarbon species, some of which form aerosols. 
These hydrocarbons strongly absorb UV and blue light on Jupiter and Saturn. 
Hydrocarbon hazes are also crucial components of the atmosphere of Titan and likely 
were important at some periods in the atmosphere of the Archean Earth. In addition, 
giant exoplanets somewhat warmer than Jupiter will likely host an array of 
photochemical S, N, and O species. Lab results point to the likely ubiquity of 
photochemical hazes under diverse conditions (Hörst et al. 2018). Any complete 
characterization of an exoplanet atmosphere should account for the presence of 
hazes. Studies of hazy giant planets in reflected light would provide a valuable 
proving ground for understanding photochemical processes in atmospheres that differ 
from those found in the solar system and would give insight into habitable exoplanet 
haze processes.  
Understanding the diversity of photochemical outcomes for all types of planets is a 
challenge. Because they form at low stratospheric pressures and are small, haze 
particles are most apparent from their interaction with blue and UV light. 
Investigating how hazes relate to atmospheric thermochemical structure, and surface 
composition/processes for terrestrial planets, will be a complex task, but one that 
will greatly inform our understanding of such processes as a whole. 
Conclusions  
The path forward for characterization of potentially habitable worlds through direct 
imaging proposed here mimics in many ways the path that has developed in the study 
of transiting exoplanets. Observational strategies, data reduction and interpretation 
techniques, and atmospheric modeling efforts were greatly refined on a sizeable 
(>100) population of giant exoplanets before inroads were made for transiting 
terrestrial exoplanets. It is our hope that any future facilities that aim to study 
exoplanets in detail acknowledge the criticality of exploring and learning from giant 
planets along the path towards characterizing habitable terrestrial planets.  
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Imaging Cool Giant Planets in Reflected Light: Science Investigations
  and Synergy with Habitable Planets

Imaging Cool Giant Planets in Reflected Light: Science Investigations and Synergy with Habitable Planets

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